As a winding having an insulating coat formed thereon by coating and baking an insulating paint, enameled wire has found extensive utility in numerous applications. For coil, which is one of the main uses for the enameled wire, to be capable of forming a strong magnetic field, it is necessary that the number of turns of the coil should be large and the magnitude of the electric current passed through the coil should be great. For this reason, the enameled wire used as coil is required to possess an insulating coat of sufficiently small thickness and, at the same time, to be capable of withstanding the Joule heat due to the great electric current.
Heat-resistant enameled wire possesses an enamel coat having at least one bond selected from imide bond, amide bond, and hydantoin bond. For the formation of this coat, polyester imide, polyimide hydantoin ester, polyamide imide ester, polyamide imide, and polyimide are mainly used, as described in U.S. Pat. Nos. 4,447,589, 4,497,944, 4,505,980, 4,307,226, 4,329,397, 4,269,397, 4,244,206, 4,258,155, 3,843,602, 3,817,942, 4,038,254, 3,817,921, 3,994,863 and 4,294,952, and British Pat. No. 1,392,649.
For the purpose of obviating the treatment with insulating varnish during the formation of a coil, there has been introduced a self-fusion wire which is provided on the surface thereof with an adhesive layer, as described in U.S. Pat. Nos. 3,705,909, 4,009,149, 4,334,973 and 4,273,829, and British Pat. No. 1,396,990, wherein a phenoxy resin, a polysulfone resin, and a polyimide resin which are aromatic macromolecular substances are chiefly used as the adhesive layer. However, it takes a relatively long time (some tens minutes) for any of these resins to be perfectly fused on an electric wire. Even in the case of a polyimide resin which exhibits satisfactory fusibility, there is a problem that about 50% of the adhesive layer produced by fusing under the conditions of 240.degree. C. and 30 minutes peels in an atmosphere kept at 250.degree. C., as reported Misao Wake, Industrial Materials, Vol. 30, No. 13, pp 33-37 (December 1982 issue).
In recent years, as electric devices and electronic devices are advancing toward gradual reduction in size in the field of ordinary electric wires, the electric wires distributed in such devices are steadily losing in diameter. Moreover, demand for electric wires possessing thermal stability to withstand the intense heat of automatic soldering and exhibiting satisfactory flexibility has been increasing. Under the circumstance, it has been proposed that polyethylene is coated on a copper wire and crosslinked, as in U.S. Pat. No. 4,125,739 and 3,951,871. As means of effecting the crosslinking, a method of electron-beam crosslinking has found popular recognition, as described in V. L. Lanza, Modern Plastics, Vol. 34, No. 10, p. 129 (1957). Although the polyethylene as the material for the coat has excellent heat resistance, the method itself entails the disadvantage that the process necessitates use of a large facility and the polyethylene coat on exposure to the electron beam is unevenly crosslinked and is liable to sustain a crack after a protracted use because the electron beam projected on the polyethylene coat is intercepted by the copper wire and prevented from permeating into the part of the polyethylene coat falling behind the copper wire. A method which comprises filling polyethylene with a silane compound and crosslinking the resulting mixture with water is also known, as described in H. G. Scott, Modern Plastics, March 1973. However, the formed coat does not adhere with ample fastness to the copper wire because the crosslinking occurs only in the surface portion and fails to proceed in the inner part of the coat. Since the crosslinking necessitates use of high-temperature steam, the facility used therefor is large. Moreover, since the crosslinking consumes much time, the entire process is not economical. In the coated wire produced by these crosslinking methods, since cooper wires as cores and resin layers as coats invariably adhere with insufficient fastness, peeling occurs inevitably between the polyethylene coats and the copper wires and this separation not merely lowers the overall strength of the coated wires but also accelerates the deterioration of the copper wires.
Further, in accordance with any of these methods, since the crosslinking does not proceed uniformly, part of the uncrosslinked polyethylene of the coat is melted while the coated wire is being soldered near 260.degree. C. and the molten polyethylene interferes with smooth deposition of solder and causes thermal deformation of the wire itself. Thus, the coated wire cannot be effectively distributed by continuous soldering.
The electric wires are not solely used for the transfer of electric power but utilized in fields increasingly diversified in consequence of steady advance of industries. In the field of electric wires for communication, for example, the growth of the multiplex operation resorting to digital signals is encouraging shift of popularity from city cables to toll cables connecting cities, to carrier cables, and further to coaxial cables laid between cities separated by long distances. For communication with oversea countries, submarine cables are used.
An extra-high voltage cable, for example, is expanded by the heat generated when the cable is passing a large current. When the amount of electric current decreases, the cable loses the heat and shrinks. Owing to the repeated rises and falls of the temperature of the cable, gaps and distorsions occur in the insulator and the sheath and eventually cause breakage of the cable. To cope with this trouble, there is adopted an oil filled cable (OF) which has a spiral gap inserted inside the cable and an insulating oil is placed under a certain hydraulic pressure within the gap so as to adjust thermal expansion or contraction of the cable. This cable necessitates use of a sheath of such metal as lead or aluminum to withstand the hydraulic pressure. This method, however, puts an extra weight to the cable and degrades the workability and, in consequence of deterioration of the insulating oil, induces corrosion of the sheathing material. Another method contemplates a gas filled cable (GF) which fills the gap inside the cable with an inert gas (such as nitrogen) to seal in the pressure. Undoubtedly, this method calls for very great care for the observation and adjustment of the gas pressure.
Some of the power cables are insulated with crosslinked polyethylene (CV, CE) and others are insulated with polyethylene and sheathed with polyvinyl chloride (EV). Recently, synthetic rubbers such as ethylene-propylene rubber (EPR) and styrene-butadiene rubber (SBR) have come to find growing acceptance because they have good elasticity and good water resistance, weigh little, have no possibility of yielding to galvanic corrosion, and warrant ease of installation. The rubber coating, similarly to the coating of crosslinked polyethylene, is excellent in flexibility and water resistance but deficient in adhesion to the conductor and the sheath. When subjected to repeated folding or to repeated changes of temperature, therefore, it produces gaps between the layers and consequently induces degradation of weatherability and water resistance. Further, the rubber materials are inferior to the crosslinked polyethylene material in electrical properties such as insulation resistance, dielectric strength, and high-frequency properties.